The overall goal of the planning phase of this single center trial is to complete all of the scientific planning and administrative activities required t support a proposal for a trial of a novel footwear for the treatment of painful medial knee osteoarthritis. Specific aims for this planning grant are: 1. to test recruitment strategies so as o develop a recruitment plan 2. to develop trial protocols; 3. to create a manual of operations. Approximately 60% of knee osteoarthritis affects the medial compartment, and when affected, this compartment is subjected to excessive loading as quantified by the external adduction moment across the knee. Wedges and orthotics which reduce the adduction moment and unload the medial compartment have been tested, but trials have shown no significant pain reduction compared to control. One reason why inserts and orthotics have not reduced knee pain compared with control may be because they do not have a large enough unloading effect on the knee. The therapy we propose to evaluate consists of a pair of shoes each with two large half rubber balls screwed into the plantar surface of the sole so that a person walks on a slightly spherical and mildly unstable surface. The location of the balls is adjusted depending on the needed knee unloading effect. This footwear has two mechanisms of action: 1. by altering the placement of the balls, it can alter the pattern of knee loading, markedly reducing load in the painful medial compartment and 2. by forcing the patient to walk on rubber balls, it retrains lower extremity motor control, encouraging better coordination and healthy recruitment of muscles. The novel footwear was introduced into the U.S. as APOS therapy and its efficacy has never been evaluated in a randomized controlled trial. The overall objective of this project is to carry out a 6 month randomized controlled trial to test the efficacy of this novel footwear therapy vs. sham in persons with painful medial knee OA. There are two goals of a trial testing APOS therapy: 1. to evaluate the efficacy of this treatment and 2. to determine what measures of performance are affected by APOS to provide evidence that targeting them might be effective for treatment of knee OA.